Time for Dividing

Late winter and early spring are ideal times to divide perennials on Alcatraz. As we do not (typically) receive frost, plants never go fully dormant as in northern climates, but herbaceous plants do slow their growing of new leaves. This window is perfect for dividing bearded iris. The plants have not yet put valuable stored energy into producing new leaves, and instead can expend energy into forming new feeder roots once it has been replanted. Once established, new leaves are produced. We have found that mature clumps of iris will still flower the same year that they were transplanted, but smaller pieces of an iris rhizome may take up to two years to flower.

Generally, we aim to divide our iris

Overgrown clump of bearded iris. Photo by Melissa Harris.

every three years, just like the Ruth Bancroft Garden does with Ruth’s heirloom collection of iris. Happy iris become overgrown and the thick rhizomes start to crowd each other, growing over top of one another. Overgrown iris can lead to several problems – poor air circulation which increases rust on the leaves, the roots competing for nutrients in the soil, and the centers of the iris clump will become bare of leaves and not produce any flowers at all. It’s easy to tell when you should take on the project of dividing your own iris if you look for these signs.

The garden volunteers divided the tall scented bearded iris in the Prisoner’s gardens this week. Four separate patches of iris were divided, and we ended up with not only the beds replanted, but with five bins of extra iris!

Bins of extra iris rhizomes. Photo by Shelagh Fritz

The iris bloomed really well last year, but as we add fresh compost to enrich the soil, the rhizomes were becoming buried. Iris likes to be planted very shallow, with the backs of the rhizome sitting above the soil.

One of the volunteers showed me an interesting feature about the rhizomes that I didn’t know before. Looking at the underside where the roots grow from, holes are visible. These were where the roots had grown from. The rhizome grows from one end, and the older end becomes a storage unit for energy (much like a potato). When dividing iris, the older sections are broken away and only the piece with the roots are kept. We are curious to see if the older section will sprout roots, so we placed a few in a pot in our greenhouse to see what happens

Holes from old roots are visible on the underside of the iris rhizome. Photo by Shelagh Fritz

The growing point is on the right, the old part without roots can be seen on the left. Photo by Shelagh Fritz

Now we are busy trying to find new homes for the divided extras, these are my favorites and I can’t bear to compost them.